The present invention relates to apparatus for measuring the apparent surface voltage on an electrostatic/electrophotographic medium, and more particularly to such apparatus incorporating calibration means to compensate for changes in temperature, relative humidity and electrical circuit performance which affect accurate apparent surface voltage measurements.
In recent years the data processing industry has used electrostatic/electrophotographic techniques, commonly used for photocopying, to provide high-speed, high-quality printers for hard-copy computer output. For example, laser printers have been developed in which the digital data corresponding to a desired computer output is used to modulate a laser beam that is scanned across an electrophotographic drum, with corresponding printed output being obtained by standard electrophotographic developing techniques. Such electrophotographic printing apparatus offer significant advantages over mechanical printers in terms of speed, print quality and reliability. Moreover, electrophotographic printing techniques offer greater flexibility in terms of font and graphics capabilities than other types of printers.
A principal design problem in adapting electrophotographic techniques to high-volume computer output printing is maintaining output print quality, in particular because many data processing applications require output printing operations that function relatively unattended. The two principal criteria for print quality are (a) print darkness (contrast), and (b) background cleanness. Electrophotographic copiers typically provide output copies that vary in print quality both in terms of print darkness and background cleanness, and users are generally tolerant of such variations. In the case of electrophotographic printers, however, users expect printed output to be of a consistently high-quality. In particular, users expect printed output to present a clean background without the smudges or streaking commonly associated with photocopies; given a clean background, users are fairly tolerant of variations in print darkness.
One of the most important parameters affecting electrophotographic print quality is the apparent surface voltage (ASV) on the electrophotographic imaging medium--for example the photoconductive surface of an electrophotographic drum. ASV is a measure of the electrical charge density on the photoconductive surface, the magnitude of which governs the optical density of the developed image (i.e., print output). In accordance with well known electrophotographic processes, the ASV on the photoconductive surface of a rotating drum is controlled using electrical corona discharge and optical exposure to provide an electrostatic image of the desired output.
To illustrate, one commercially available electrophotographic apparatus includes (a) a first positive corona that initially charges the photoconductive surface to a high positive potential, or ASV, on the order of 2,000 volts, and (b) a second negative corona that provides a negative discharge as corresponding portions of the photoconductive surface are optically exposed. As a result, ASV is reduced to a positive 400 to 500 hundred volts in optically unexposed areas, and -60 to -90 volts in exposed areas, thereby forming an electrostatic image. The positive ASV corresponds to the desired print image while negative ASV corresponds to unprinted background, so that the electrostatic image is developed by bringing negatively charged toner particles into contact with the photoconductive surface, where they adhere to the positive ASV print image for ultimate transfer to paper.
To provide quality print output, the ASV levels on respective portions of the photoconductive surface corresponding to print image and background should be within well defined ranges. Thus, for the above exemplary electrophotographic process, the ASV for the print image should be on the order of a positive 400 to 500 volts, while background ASV should be in the range of -60 to -90 volts. In particular, if background ASV is less negative than -60 volts, the negatively charged toner particles will not be sufficiently repelled so that some will adhere to the background image portion of the photoconductive surface, causing the smudging or streaking commonly associated with photocopies. On the other hand, if background ASV is more negative than -90 volts, then, in the case of the common toner/carrier particle developing technique, some of the positively charged carrier particles, along with adhering negative toner particles, will be transferred to the photoconductive surface resulting in a splotched or pitted background.
The appropriate ASV levels on a photoconductive surface are established by controlling corona discharge amplitudes and optical imaging intensities. Unfortunately, for a given corona amplitude and optical intensity, ASV is subject to time variation due to numerous uncontrollable causes such as changes in temperature, relative humidity and electrical circuit performance, as well as the hysteresis effects of pre-existing ASV. Indeed, these environmental and electrical factors are primarily responsible for the previously discussed variations in the output quality of electrophotographic processes.
The environmental and electrical conditions that affect ASV are too complicated to control directly. Thus, current approaches to eliminating or reducing their affect on output print quality focus on controlling ASV by continuously adjusting either optical image intensity or corona amplitude. That is, for the above exemplary electrophotographic process, image quality can be controlled by periodically measuring changes in ASV due to environmental and electrical factors, and then compensating for these factors by adjusting, typically, optical image intensity to maintain the background ASV in the exposed portions of the photoconductive surface within the desired range (i.e., -60 to -90 volts). While this compensation scheme would not affect the positive 400 to 500 volt ASV of the unexposed (printed image) portions of the photoconductive surface, it would insure a clean background for the printed output.
ASV measurement is typically accomplished using a capacitively coupled or ammeter-type sensing probe comprising a sensing head disposed adjacent a portion of the photoconductive surface, together with suitable probe electronics to convert sensed ASV into a probe output voltage. Accurate ASV measurements, however, are not straightforward because the same factors that affect ASV--temperature, relative humidity and electrical circuit performance--also affect probe sensitivity (i.e., the relationship between sensed ASV and probe output voltage). Typically, fairly complex electronics and/or manual calibration schemes are provided to reduce the effects of these factors on ASV measurement.
In addition, because the probe sensing head is AC coupled to the probe electronics, the output probe voltage is subject to drift/offset effects. To eliminate these effects, differential ASV readings must be taken by periodically exposing the probe to a known reference voltage (typically ground). Current methods of providing a ground reference voltage include interposing a grounded tuning fork or oscillating diaphragm between the sensing head and the photoconductive surface such that the probe is periodically exposed to a ground reference.
The current techniques for compensating for the environmental and electrical factors that affect ASV measurement, and for providing a known reference voltage to eliminate electronic drift/offset effects, make presently available ASV probes that are accurate enough for measuring ASV in an electrophotographic printing apparatus complex devices costing hundreds or thousands of dollars. As a result, these ASV probes are not cost effective to incorporate into low-to-medium cost electrophotographic printing apparatus.